The present invention relates to a method, a device and a control unit for checking the performance reliability of a timer which is arranged outside of the control unit, used in particular for controlling an internal combustion engine, and which provides the control unit with time information, time information being generated internally in the control unit and being compared to corresponding time information supplied by the external timer.
Time information can be needed in various ways by a control unit. For controlling internal combustion engines, the knowledge of the time duration after the engine is switched off until the start once again is advantageous for controlling the further driving process. Depending on the length of the engine shutdown time, the injection time for the start can be corrected, the model for the exhaust-gas temperature can be pre-initialized accordingly, and catalytic-converter heating measures can be taken or forestalled. Furthermore, in the case of long shutdown times, automatic diagnostic tests can be enabled.
The shutdown time can be calculated from three different information sources. Short shutdown times can be yielded from a time measurement during the control-unit after-running. For long shutdown times, calculation models exist which ascertain the shutdown time from the cooling off of the engine (German Published Patent Application No. 19644497). Finally, to ascertain accurate shutdown time information, it is possible to use an external timer, e.g. an on-board clock located outside of the control unit. To that end, the control unit is designed such that, at desired points of time, the time data of the on-board clock are read out and subsequently processed. Such a method is described in German Published Patent Application No. 196 21 900.
German Published Patent Application No. 196 37 088 describes a control system for a motor vehicle, in which the instant the engine is switched off is transmitted by the on-board clock via a CAN data bus to the engine control unit. The same holds true for the instant of re-igniting the engine, so that the engine shutdown time can be calculated from the time difference, and the catalytic-converter temperature can be determined as a function of this engine shutdown time. A plausibility check of the time information supplied by the on-board clock is not carried out here.
A comparison of the shutdown time, calculated from the external clock information, to the shutdown time calculated from the engine cooling is too imprecise for a plausibility check, particularly in the case of brief shutdown times. In addition, the operation of block heaters or auxiliary heating can strongly influence the shutdown time calculated from the engine cooling.
German Published Patent Application No. 196 52 645 describes a method of this type for checking the performance reliability of an external timer. In that case, a software timer in the control unit is started with a specifiably defined time interval, and the external time information (e.g. of an on-board clock) is read out at the beginning and at the end of this time interval. The external time interval ascertained from this is subsequently compared to that of the software timer. If a deviation is determined which exceeds a specifiable extent, a malfunction of the external timer is detected.
A disadvantage of the known method is that the plausibility check is carried out only once in relation to the relatively brief time interval of the software timer. In contrast, the objective in the present case is to specify an improved method with which, in particular, the plausibility check is carried out with higher accuracy.
In the method according to the present invention, an internal time meter is started in the control unit, and the time information supplied, first of all, by the time meter internal to the control unit, and secondly by the external timer is compared at arbitrarily specifiable points of time. This has the advantage that a plausibility check can be carried out during the entire operating time of the control unit and specific to the total operating time. Thus, it is possible both to detect a fault of the external timer at any time during the operating time, as well as to increase the accuracy in calculating the deviation.
To this end, it is advantageous to carry out the plausibility check over the entire operating time of the control unit, i.e. of the internal time meter. First of all, this allows conclusions about the performance reliability of the external timer during the entire engine operation. Furthermore, short-duration disturbances, for example, during the transmission of the time information from the external timer to the control unit, can be ascertained during the fault analysis, since the check is carried out over the entire period of time of the engine operation.
It is advantageous to start the time meter, internal to the control unit, when the ignition of the internal combustion engine is switched on, in order to immediately check the time information, supplied by the external timer, for plausibility.
Furthermore, directly after the start of the internal time meter (for example, after ignition has been effected), the signal supplied by it can be checked for the presence of time information in order to be able to immediately detect a complete failure. The check for the presence of time information can be carried out independently of the fact that the internal time information is compared to the external time information (in so far as existing).
The plausibility check is carried out in a simple manner, for example, in that predetermined time segments which are defined by the internal time meter are in each case compared to the corresponding time segments derived from the external timer. It is also possible to constantly form the ratio of the total time spans since the switch-on of the control unit (or another defined point of time, e.g. engine start) of the internal and external time information. To this end, the corresponding signal of the external timer is advantageously supplied to the control unit and further processed there. For example, the deviation is calculated, or a ratio of the corresponding time segments is formed, the external time information being recognized as faulty when the deviation exceeds a specific threshold value or lies outside of a predetermined interval. It is then useful to enter corresponding fault information into a fault storage. When the engine is started again, the fault information can then be read out from the fault storage, and the determination of the shut-down time can be omitted. A new plausibility check can subsequently be carried out, to be able to determine whether the fault of the external timer was eliminated.